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An Auger Electron

Spectroscopy Study

of Surface_PreparationContaminants

D. Wu,

R. M. Stephens, _R. A. Outlaw,

and P. Hopson__

AUGER FLFCT R,ON

SURe:ACE-PREPARATION

16 p CSCL 13B

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NASATechnicalPaper2972

1990

National Aeronautics andSpace Administration

Office of ManagementScientific and TechnicalInformation Division

An Auger Electron

Spectroscopy Studyrof Su face-Preparauon

Contaminants

D. Wu

Old Dominion University

Norfolk, Virginia

R. M. Stephens,

R. A. Outlaw,

and P. Hopson

Langley Research Center

Hampton, Virginia

Abstract

Many cleaning methods are presently beingemployed for the surface preparation of materials

that are subsequently exposed to ultrahigh vacuum(UHV). Unfortunately, there are virtually no com-parative measurements that establish the residualcontaminant level of each method. In this report,11 different cleaning methods, ranging from only de-tergent cleaning to electrochemical polishing, havebeen applied to 12 identical samples of 347 stainlesssteel. Two surface conditions, a standard machinedsurface and a mechanically polished surface, havebeen studied. Auger electron spectroscopy (AES)within a UHV environment has been used to de-

tect the types of contaminants and the magnitudesfound on the sample surfaces. It has been found that

electrochemical polishing gave the least contami-nated surface for all metals studied and that me-

chanically polished surfaces were significantly cleanerthan the as-machined surfaces for any given cleaningmethod. Furthermore, it was also found that theresidual contaminants left by methanol, ethanol, iso-propyl alcohol, acetone, and Freon finishing rinseswere almost the same.

Introduction

One of the most significant problems encounteredin preparing surfaces for ultrahigh vacuum (UHV) isthat of residual contamination. This contamination

level (especially when large surface areas are consid-ered) can substantially affect the ultimate pressurethat is obtainable, even following a bakeout. Thereare many different cleaning methods presently usedin research laboratories (refs. 1 to 9), but most re-searchers depend on their intuition to judge the ef-fectiveness of the method since very few comparisonstudies have been made.

In this study, comparisons are made of the con-tamination on the surfaces of 12 identical samplesof 347 stainless steel with two different surface con-

ditions, a standard machined surface (considered"rough" and designated by "B") and a mechani-

cally polished surface (considered "smooth" and des-ignated by "A"). These samples were cleaned by11 different methods that are often used in research

laboratories as a standard practice for the prepara-tion of materials that are to be within a UHV en-

vironment. Auger electron spectroscopy (AES) wasemployed to evaluate and compare the residual Con-tamination levels of the different methods. The sen-

sitivity of this method is less than 10 -2 (1 atomicpercent) for every elemental aggregate (except hydro-gen). This method is also limited by the detection ofAuger electrons other than those from the top sur-

face layer since the inelastic mean-free path for sometransitions is greater than 10/_.

Experimental Apparatus and Procedure

Sample Preparation

The 347 stainless steel samples were cut to a

disk geometry with a 10-mm diameter and a 1-mmthickness and were prepared in the following ways.

SamplesA1 toA12. Samples A1 to A12 were ma-chined to a 32-pin. finish and were mechanically pol-ished. The samples were mounted on a metal blockby using wax and were polished with successivelysmaller sized abrasive materials (SiC/H20:600 gritand 5-#m grit; and silica salts: 0.05 #m). After pol-ishing, the samples were rinsed in acetone to removethe wax and were then rinsed in deionized water.

Samples B1 to B12. Samples B1 to B12 weremachined to a 32-#in. finish.

Surface Preparation

The sample surfaces were prepared, in general, forUHV applications using UHV techniques. In all cases(where possible), reagent-grade (high-purity) chem-icals were used. The preparations extend from the

condition of no cleaning (as-machined or polished)to 11 different cleaning procedures that involve themost often used combinations of detergents, solvents,and acids; this was followed by deionized H20 or sol-vent rinses. Simultaneous stirring or ultrasonic agita-tion with the cleaning procedures was also employed.The details of these procedures can be found in theappendix.

Apparatus and Procedure

The experimental apparatus for Auger electronspectroscopy (AES) is shown in figure 1. The sam-ples were introduced into the AES system within1 hour after cleaning in order to minimize the con-tamination from the environment and provide someconsistent level of atmospheric exposure. The sam-ples were individually mounted on a sample holderwhere the surface normal is inclined 60 ° to the ana-

lyzer axis. A schematic of this apparatus is shown infigure l(a). Several specimens were introduced intothe system load lock and mounted on the carousel

holder at each loading, and then they were sequen-tially r6tated into the analysis position. The sampleswere exposed to UHV (Pressure < 5 x 10 -10 torr) forabout 24 hours, and then the analysis was conducted.The AES data were obtained with a cylindrical mir-ror analyzer (CMA) operated with a coaxial gun atan energy of 3 keV and a beam current of 0.5 #A.

The angle of incidence was 60° to the specimen sur-face normal. Room temperature AES spectra weretaken at three different randomly selected locations

• on the surface. The average intensity for each ele-ment was then normalized by the iron LMM Auger

transition intensity at 651 eV.

Results and Dlscfission

The surface of the samples following the 12 sur-face treatments (see the appendix) was mainly con-taminated with oxygen, carbon, and sulfur. Othersurface contaminations included phosphorus, potas-sium, silicon, calcium, chlorine, and nitrogen. Thesenormalized AES peak intensities are presented in ta-ble 1 for the smooth surface and in table 2 for the

rough surface. The values in parentheses are thesame data except that the relative Auger sensitivityof the elements has been taken into account. Fig-ure 2, which presents electror;energyplotted againstthe derivative AES spectra dN(E)/dE, shows a

sputter-cleaned 347 stainless steel sample that ispresented for reference as a clean, contaminant-freesurface.

The spectra for samples B1 and A1 (both nocleaning) are shown in figures 3 and 4, respectively.Figure 3 (sample B1) shows that the machined sur-face was mainly contaminated by carbon, sulfur, and

oxygen, probably from residual machine oil. As isevident from the substantial reduction in AES sig-nal intensities, figure 4 (sample A1) shows that me-Chanical polishing reduced the surface porosity andinclusions (and therefore the surface area), thus min-imizing the traps for oil and other contaminants. Theoxygen signal is not decreased by the polishing be-cause normal room-temperature oxidation occurs onboth surfaces following the machining and polishing.Furthermore, the oxygen signal is larger than that in

figure 3 only because of the heavy carbon overlayer.Samples A2/B2 (A2 and B2) and A3/B3 (A 3

and B3) were all prepared with the same degreas-ing condition and were finished with deionized water,but A2/B2 were prepared without ultrasonic agita-tion and A3/B3 were prepared with ultrasonic agita-tion. Samples A4/B4 and A5/B5 were prepared withthe same degreasing conditions and were finishedwith methanol, but A4/B4 were prepared withoutultrasoni_ agitation and A5/B5 were prepared withultrasonic agitation, it was found that for a smoothsurface (the A samples), the contamination level lefton the surface was virtually the same for samplesboth with and without ultrasonic agitation, but for

a rough surface (the B samples), the contaminationlevel of oxygen, carbon, phosphorus, and chlorine wassignificantly reduced With ultrasonic agitation com-pared to that without ultrasonic agitation. This may

2

be because ultrasonic cleaning vibrates off most ofthe particulants on a rough surface and enhancescleaning of the mechanical grooves. An exceptionwas noticed with methanol finishing (for both smoothand rough surfaces) in that ultrasonic cleaning left ahigher carbon concentration compared to that with-out ultrasonic cleaning. This is somewhat-surpris-ing, but we conducted this experiment many times,and always observed the same behavior. Perhaps theadded energy of the ultrasonic agitation promotedcarbon bonding to the surface.

Samples A3/B3 to A9/B9 were prepared withthe same degreasing conditions but with differentfinishing rinses (methanol, ethanol, isopropyl alcohol,acetone, and Freon 1). A comparison of these differentfinishing rinses for both smooth and rough surfacesShowed that the O/Fe ratio is almost the same forall the specimens, which is as expected. For a roughsurface, acetone finishing has the lowest C/Fe and

S/Fe ratios, but for a smooth surface, acetone hasthe highest C/Fe and S/Fe ratios. This result maybe explained by considering that acetone is the moreefficient solvent for removing oils and contaminants

from the rough surface, but on a smooth surfacethe residue left by the acetone is greater than thatfor the other solvents. A methanol/ethanol finishing

rins_e ha s the lowest C/Fe and S/F e ratios on smoothsurfaces but the highest on rough surfaces, whichprobably indicates that large-molecule solvents likeacetone and isopropyl alcohol, as compared withethanol and methanol, are more efficient in removingoils and other contaminants from the surface but

leave more residual contaminants from the impuritiesin the solvent itself. The reason for this effect

may be the higher physical interaction potential forcontaminant molecules.

Samples A10/B10 to Ail/Bll were preparedwith detergent degreasing plus chemical cleaning.The representative spectra are shown in figures 5and 6. A comparison between these stl-ong acidcleaning methods shows that the dlfference betweensmooth and rough surface samples is small, quiteprobably because acid etching removed most of thecontamination. Comparing the Varian Associatesrecipe with the Stanford linear accelerator (SLAC)recipe for samples with a rough surface shows thatthe Varian recipe has a higher C/Fe ratio but theSLAC recipe has a much higher O/Fe ratio, evenhigher than that for ordinary alcohol finishing meth-ods. It was also observed that the SLAC recipe has

higher P/Fe, Si/Fe, and Ca/Fe ratios than the Varianrecipe, probably the result of the alkaline soak after

1 Freon: Registered trademark of E. I. du Pont de Nemours &Co., Inc.

theacidcleaning.Theimpuritylevelof thealkalinesoakdid affectthefinalcontaminationofthesurface.Wealsonoticedthat the chlorineconcentrationforthe SLACrecipeis higherthan that for the Varianrecipe.

Electrochemicalpolishing (samplesA12/B12)providedthe cleanestsurfaceof all methods,partlybecauseof thepolishingthat reducedthesurfaceareaandthechemicalremovalof the contaminants.Thechilledmethanol-perchloricbath formeda passivat-ing chlorine-bearinglayeron the top of thesurface,ascanbe seenfrom figure6. The nickelpeakat848eVandthechromiumpeakat 529eV havedis-appearedcompletely.Thesearevalencetransitionssuggestingthat thenickelandchromiumhaveformedachemicalbondwith thechlorineandthattheresult-ing chlorine-bearinglayeron the surfacemayhavea passivatingeffectthat inhibitsadsorptionof otherspecies.However,if chlorineisanundesirablespecie,this maynot bea preferredcleaningmethod.Fig-ures7 and8 showbargraphrepresentationsof ta-bles1 and2 for thesmoothandroughsurfaces,re-spectively.Thetotal intensitiesof thecontaminantsgivea generalindicationof the effectivenessof thesurfacepreparation.

Finally,in all theaforementionedprocedures,thequality of the solventsandchemicalswasfoundtobe important. Reagent-gradechemicalsproducedthe leastcontaminatedsurface(for a givenrecipe),whereaschemicalstakenfromindustrialcontainers,suchasa500-galdrum,werethemostcontaminated.Asonewouldexpect,thecleanlinessofthelaboratorybeakersandcontainersusedin theserecipeswasalsoa majorfactor.

Concluding Remarks

Comparisons have been made between differentcleaning methods often used in the research labora-tory as a surface preparation for ultrahigh vacuumsystems. The results of this comparative analysisshowed that mechanical polishing is a very effectiveway to reduce the level of surface contamination.The results of the smooth surface without any pre-cleaning were better than the results of the rough sur-face with chemical cleaning. For a smooth surface itwas observed that a methanol/ethanol finishing rinseis better than an isopropyl alcohol/Freon/acetonefinishing rinse, but for a rough surface the latterrinse is superior. The probable reason for this isthat the surface area for a smooth surface is much

Smaller, and the residue from a large-molecule sol-vent like acetone and isopropyl alcohol is competitivewith the residual contamination left from the polish-ing. On the other hand, there is'a greater amount

of carbon-bearing contaminants being trapped in theinclusions and general surface defects on a rough sur-face for which the large-molecule solvent is more ef-ficient. Ultrasonic cleaning is also a useful techniqueto improve the efficiency of a given cleaning methodfor a rough surface. These results also showed thatchemical cleaning can remove most of the carbon andsulfur and is significantly better than just detergentcleaning plus solvent rinse finishing.

From this analysis it appears that electro-chemical polishing and mechanical polishing aresuperior methods for preparing surfaces, but when

polishing is not possible, the surfaces should bechemically cleaned.

NASA Langley Research CenterHampton, VA 23665-5225January 22, 1990

Appendix

Cleaning Method

A1/BI:No cleaning.

A2/B2:1. Cleaned by agitated detergent solution for

5 minutesat a temperature ofl90°F:2. Rinsed by agitated deionized water for 10 min-

utes at a temperature of 190°F.3. Rinsed by agitated deionized water for 5 min-

utes at a temperature of 190°F.4. Dried by hot air.

A3/B3:1. Ultrasonically cleaned by detergent solution

for 15 minutes at a temperature of 190°F.2. Ultrasonically rinsed by deionized water for

10 minutes at a temperature of 190°F.3. Ultrasonically rinsed by deionized water for

5 minutes at a temperature of 190°F.4. Dried by hot air.

A4/B4:1. Cleaned by agitated detergent solution for

15 minutes at a temperature of 190°F.

2. Rinsed by agitated deionized water for 10 min-utes at a temperature of 190°F,

3. Rinsed by agitated deionized water for 5 min-utes at a temperature of 190°F.

4. Rinsed by agitated methanol for 5 minutes.5. Dried by hot air.




5 minutes at a temperature of 190°F.4. Ultrasonically rinsed by methanol for

5 minutes.

5. Dried by hot air.




5 minutes at a temperature of 190°F.4. Ultrasonically rinsed by ethanol for 5 minutes.


.

3.

4.


for 15 minutes at a temperature of 190°F.Ultrasonically rinsed by deionized water for10 minutes at a temperature of 190°F.Ultrasonically rinsed by deionized water for5 minutes at a temperature of 190°F.Ultrasonically r]_y isopropyl alcohol for5 minutes.





5 minutes at a temperature of 190°F.4. Ultrasonically rinsed by Freon for 5 minutes.5. Dried by hot air.


for 15 minutes at a temperature of 190°F.

2. Ultrasonically rinsed by deionized water for10 minutes at a temperature of 190°F.

3. Ultrasonically rinsed by deionized water for5 minutes at a temperature of 190°F.

4. Ultrasonically rinsed by acetone for 5 minutes.5. Dried by hot air.

AIO/BIO (recipe from Varian):1. Degreased by vapor in trichloroethylene for

5 minutes.2. Rinsed by deionized water for 1 minute.

3. Cleaned by agitated detergent for 5 minutesat a room temperature of 190°F.

4. Rinsed by hot deionized water for 2 minutes.5. Acid cleaned by 33 percent HNO3, 33 percent

HF, 33 percent H20 bath (by volume) at roomtemperature for about 20 sec.

6. Rinsed by hot deionized water for 2 minutes.7. Dried by hot air.

All/Bll: (recipe from SLAC):1. Degreased by vapor in hot trichloroethane va-

por for 5 minutes.2. Rinsed by cold tap water for 1 minute.3. Cleaned by alkaline soak for 5 minutes at a

temperature of 190°F.4. Rinsed by cold tap water for 2 minutes.5. Immersed in a stainless steel pickle consisting

of:

(a) 1 part nitric acid (42 Baum@)(b) 1 part hydrofluoric acid (48 percent)(c) 1 part water

4

(d) Temperature:room(e)Time:until part is freefromscaleand

oxide(about10sec)

6. Rinsedbycoldtapwaterfor 2minutes.7. Cleanedby alkalinesoakfor 5 minutesat a

temperatureof 190°F.8. Rinsedby coldtap waterfor 2minutes.9. Immersedin 25 to 30percentnitric acid(by

volume)for 2 minutes.10.Rinsedby coldtap waterfor 2minutes.11.Rinsedby colddeionizedwaterfor 2 minutes.12.Rinsedby hotdeionizedwaterfor 2 minutes.13.Rinsedby isopropylalcoholat a temperature

of l15°F.

A12/B12 (electrochemicallypolished):1. Ultrasonicallycleanedbydetergentfor 15min-

utesat a temperatureof 190°F.2. Ultrasonicallyrinsedby deionizedwater for

10minutesat a temperatureof 190°F.3. Ultrasonicallyrinsedby deionizedwater for

5 minutesat a temperatureof 190°F.4. Electrochemicalpolishing:

(a)6 percentperchloricacid(70percent)(b) 94percentmethanol(c) Temperature:-70°C(d) Electricalcurrent:0.2A/cm2(e)Time: about20sec

5. Rinsedby deionizedwater.6. Driedby hot air.

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References

1. Mathewson, A. G.: The Surface Cleanliness of 316 L + N

Stainless Steel Studied by SIMS and AES. Vacuum,

vol. 24, no. 10, Oct. 1974, pp. 505-509.

2. Mathewson, A. G.; Bacher, J.-P.; Booth, K.; Calder, R. S.;

Dominichini, G.; Grillot, A.; Hilleret, N.; Latorre, D.;

Le Normand, F.; and Unterlerchner, W.: Comparison of

Chemical Cleaning Methods of Aluminum Alloy Vacuum

Chambers for Electron Storage Rings. J. Vac. Sci.

Technol., vol. A7, no. 1, Jan./Feb. 1989, pp. 77-82.

3. Bevolo, A. J.; Beaudry, B. J.; and Gschneidner, K. A.,

Jr.: Auger Analysis of the Passivation of Gadolinium by

Electropolishing. J. Electrochem. Soc., vol. 1127, no. 12,

Dec. 1980, pp. 2556-2557.

4. Rosebury, Fred: Handbook of Electron Tube and Vacuum

Techniques. Addison-Wesley Publ. Co., Inc., c.1965.

5. Blue, G. D.; and Moran, C. M.: The Effects of Sur-

face Pretreatment and Nitrogen Tetroxide Purification on

the Corrosion Rate of Type 304L Stainless Steel. 1985

Jannaf Propulsion Meeting, Volume I, CPIA Publ. 425,

VOL. I (Contract N00024-85oC-5301), Applied Physics

Lab., Johns Hopkins Univ., 1985, pp. 155 157.

6. Moore, George: Auger Evaluation of Plasma Cleaning of

Ceramics and Metals. J. Environ. Sci., vol. 25, no. 5,

Sept./Oct. 1982, pp. 33 35.

7. Emerson, L. C.; Clausing, R. E.; and Heatherly, L.:

Surface Impurity Studies in the ISX-A Tokamak. J.

Vac. Sci. _ Technol., vol. 16, no. 2, Mar./Apr. 1979,

pp. 766-768.

8. Johnson, Thomas M.: Chemical Cleaning of 304 and

304-L Stainless Steel Surfaces by the Nitradd Passiva-

tion Process. MLM-3200 (Contract No. DE-AC04-76-

DP00053), Monsanto Research Corp., Nov. 27, 1984.

9. Kohl, Walter H.: Handbook of Materials and Techniques

for Vacuum Devices, Reinhold Publ. Corp., c.1967.

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: :: Figure 1. Experimental apparatus for Auger electron spectroscopy (AES).

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Figure 7. Bar graph of sum of contaminant intensities for smooth (polished) surface. Sensitivity correctionsare included. (See table 1 for further clarification.)

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15

_ _ r v_i _ _ _ Ii_ r _

Report Documentation PageNat,o,na{ Aeronaut,cs and

Space Adm, n+stratLor_

3. Recipient's Catalog No.1. Report No. / 2+ Government Accession No.

NASA TP-2972 t4. Title and Subtitle

An Auger Electron Spectroscopy Study of Surface-PreparationContaminants

7. Author(s)

D. Wu, R. M. Stephens, R. A. Outlaw,and P. Hopson

9. Performing Organization Name and Address

NASA Langley Research CenterHampton, VA 23665-5225

12. Sponsoring Agency Name and Address

National Aeronautics and Space AdministrationWashington, DC 20546-0001

5. Report Date

February 1990

6. Performing Organization Code

8. Performing Organization Report No.

L-16653

10. Work Unit No.

763-01-41-53

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Paper

14. Sponsoring Agency Code

15. Supplementary Notes

D. Wu: Old Dominion University, Norfolk, Virginia.R. M. Stephens, R. A. Outlaw, and P. Hopson: Langley Research Center, Hampton, Virginia.

16. Abstract

Many cleaning methods are presently being employed for the surface preparation of materialsthat are subsequently exposed to ultrahigh vacuum (UHV). Unfortunately, there are virtually nocomparative measurements that establish the residual contaminant level of each method. In thisreport, 11 different cleaning methods, ranging from only detergent cleaning to electrochemicalpolishing, have been applied to 12 identical samples of 347 stainless steel. Two surface conditions,a standard machined surface and a mechanically polished surface, have been studied. Augerelectron spectroscopy (AES) within a UHV environment has been used to detect the typesof contaminants and the magnitudes found on the sample surfaces. It has been found thatelectrochemical polishing gave the least contaminated surface for all metals studied and thatmechanically polished surfaces were significantly cleaner than the as-machined surfaces for anygiven cleaning method. Furthermore, it was also found that the residual contaminants left bymethanol, ethanol, isopropyl alcohol, acetone, and Freon finishing rinses were almost the same.

17. Key Words (Suggested by Authors(s))

AugerSurface contaminants

Vacuum degreasing

18. Distribution Statement

Unclassified--Unlimited

Subject Category 31

19. SecurityClassif. (of this report)Unclassified [20. SecurityClassif.(of this page)Unclassified 121' N°" °f Pagest 22' Price16 A03

NASA FORM 1626 OCT 86

For sale by the National Technical Information Service, Springfield, Virginia 22161-2171

NASA-Langley, 1990

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